The present invention relates to apparatus for selectively transferring data packets between a plurality of commercially available local area networks (LANs), e.g., Ethernet, token ring, etc. Such commercially available apparatus, referred to as bridges or switches, typically use separate source address tables for identifying data sources on each LAN and then selectively transfer data packets between LANs depending upon the contents of the source address tables. Ideally, when a data packet is directed to a destination device on the same LAN, the data packet is not transferred to any of the other LANs and only when the destination device is known to be located on another LAN or when the location of the destination device is not known are data packets transferred to another LAN. Thus, data packet traffic is ideally limited to only the required LANs.
Many such commercially available switches attempt to maintain full-wire speed, the ability to transfer data packets between any two discrete LANs without any speed restrictions, e.g., lost or dropped data packets. Additionally, many switches, commonly described as cut through switches, attempt to minimize the time delay between starting to receive a data packet on a first LAN and starting to retransmit that data packet on a second LAN. However, if transmission starts before reception is completed, the possibility exists that an invalid data packet, e.g., a data packet with CRC errors, will then be transferred only to be to discarded by all of the devices on that LAN. When both of these design restrictions are concurrently applied to a switch, the potential architectures for such apparatus is restricted, increasing complexity and costs.
U.S. Patents directed to apparatus for switching or bridging LANs include 4,627,052; 4,715,030; 4,737,953; 4,922,503; and 5,136,580.